Relative humidity meter



211:; 118, 1942. F, KAHN RELATIVE HUMIDITY METER Filed Sept. l0, 1938 4 Sheets-Sheet l w w. w w n w w w.

Fly 7 Fay. 3.

INVENTOR Aug. i8, 1942. F. KAHN RELATIVE HUMIDITY METER Filed Sept. l0, 1938 4 She-elis-Shea?l 2 Fig, 4.

|NVENTOR Zub/L4M fig. /4

Aug- 18 E942 F. KAHN 2,293,064

RELATIVE HUMIDI TY METER Filed Sept. lO, 1938 4 Sheets-Sheet 3 2;: 246 20s 20a F2- 20a o1 Hg 20 2 206 20a 0l 202 INVENTOR 4 Sheets-Sheet 4 F. KAHN RELATIVE HUMIDI TY METER Filed Sept. l0, 1938 @mi u ug. i8, 1942.

INVENToR Patented Ang. ii, i942 RELATIVE i 15 Claims.

This invention relates to hygrometry and, more particularly, relates to a thermocouple relative humidity meter.

The most convenient and mostgenerally employed method for measuring the degree of saturation of the atmosphere with moisture is to observe the temperature of evaporation-that is, the di'erence between the dry-air and wet-air temperatures. The dry-air temperature is read with the ordinary thermometer. The wet-air temperature may be accurately observed with the sling or whirled psychrometer or with the wetair thermocouple described in Patent No. 2,128,462, dated August 30, 1938, issued to E. R. Wayne and myself and relating to thermocouple hygrometers. With the difference between the dry-air and wet-air temperatures determined, the relative humidity may be obtained from the Weather Bureau psychrometric tables, or the equivalent standard formula or chart.

'I'his method of determining relative humidity, although of high accuracy, is inconvenient and because it is not direct reading is unsuited for recording or for control of humidity conditioning apparatus. On the other hand reasonably priced direct reading devices, which have been based upon relative tension of hygroscopic ma.- terials, are highly unreliable, and require much too frequent calibrations and adjustments to render their use practical.

The increasing importance of air conditioning emphasizesthe need for a simple, compact, inexpensive and reliable device for measuring relative humidity. Apparatus for this purpose heretofore available has been extremely large and expensive, very inconvenient and laborious to operate, or highly unreliable.

The primary object oi my invention is to provide a direct reading relative humidity meter in which these defects are remedied.

An object of my invention is to produce a direct reading relative humidity meter of high accuracy.

Another object of my invention is to provide a reliable direct reading relative humidity recorder.

A further object of my invention is to produce an inexpensive, simple, durable, and reliable direct reading relative humidity indicator of good accuracy.

Another object of my invention is to provide an indicating instrument having an elastic scale.

Still another object of my invention is to provide a reliable control for humidity conditioning apparatus having a maximum of simplicity.

Still a further object of my invention is to provide a wet-air thermopile which when subjected to standard evaporatlng conditions will produce an E. M. F. which is accurately directly proportional to the difference between the dry-air and wet-air temperatures.

Another object of my invention is to provide an automatic direct-reading relative humidity meter which is accurate, occupies a minimum of space, requires very little water, and needs practically no attention or servicing.

With these and other objects in view, which will become apparent as the description proceeds, my invention is embodied in a relative humidity observing system which consists essentially of a wet-air thermocouple unit in a stream of the air being metered, an indicating or cold junction unit in the same air, a dArsonval type indicating meter in the thermocouple circuit, and means responsive to the dry-air temperature of the air being measured to cause the indication of the meter to vary as a function of the dry-air temperature.

The invention, both as to details of construction and combination of parts, will best be understood from the following description of a, speciiic embodiment which I illustrate as an example, when read in conjunction with the accompanying drawings, in which:

Fig. 1 is a diagrammatic representation of the thermocouple circuit of my invention having a single thermocouple.

Fig. 2 is a, diagrammatic representation of the thermocouple circuit of my invention having a three-junction wet-air unit.

Fig, 3 isa diagram showing the relation of relative humidity to the dry-air`temperatures and the difference between the dry-air and wetg high-accuracy relative humidity indicator according to my invention.

Fig. 5 is a perspective view of the rotatable scale of the indicator of Fig. 4.

Fig. 6 is a diagrammatic view of a remotely actuated thermo-sensitive element for rotating the scale of the device of Fig. 4.

Fig. 'l is a diagrammatic view of a modification of my invention having a resilient scale.

Fig. 8 is a diagrammatic view of another modication of my meter with resilient scale.

Fig. 9 is a diagram of a modified meter element for use in the thermocouple circuit.

Fig. 10 is a fragmentary view, showing details of part of the device of Fig. 9.

Fig. 11 is a diagrammatic view, partly in section, of another form of thermostatic member for the device of Fig. 9.

Fig. 12 is a diagram of a dArsonval meter eld magnet in which the armature flux-density is thermostatically controlled according to my invention.

Figs. 13, 14, 15 and 16 show other modifications of dArsonval meter field magnets adapted to provide temperature-controlled current-sensitivity.

Fig.- 17 is a front elevation of the self-contained direct-reading relative humidity meter of my invention.

Fig. 18 is a fragmentary front elevation of the device of Fig. 17.

Fig. 19 is a perspective view of the wet-air unit of the device of Fig. 17.

Fig. 20 is a fragmentary side elevation of a modification of the device of Fig. 17 in which the electric fan motor is replaced by a spring motor.

Fig. 21 is a diagramof a strip chart recorder for use with my relative humidity meter.

Fig. 22 is a diagram of a circular chart recorder for use with my relative humidity meter.

Fig. 23 is a diagram of `an arrangement for relative humidity control using the meter of my invention.

Referring now to the drawings, I have shown in Fig. 1 a wet-air thermocouple unit 29 comprising a thermojunction 3| formed by union of two couple elements 32 and 33 secured with good thermal contact to a thermally conducting member 34, the whole being enveloped by an evaporative envelope 35. The thermocouple electric circuit is completed through a sensitive dArsonval-type ammeter 36, which forms the reference junction and which may be constructed in the manner more fully described hereinafter with reference to Figs. 4 to 16, inclusive. When the air to be measured is forced past the wet-air unit 29 at the proper velocity with the wick 35 suitably meisten-ed and with the ammeter 36 at the dry-air temperature, the net E. M. F. of thecircuit, and therefore the current in the circuit, is substantially proportional to the depression of the wet-air temperature below the dry-air temperature.

Fig. 3 is a graph in which the unbroken lines are curves of constant relative humidity, ordinates are dry-air or ambient temperatures, and abscissae are depressions of wet-air temperature below dry-air temperature. These data are plotted from the standard psychrometric tables in the United States Weather Bureau publication W.B.No.235.

Fig. 4 shows a relative humidity indicator mounted in a case 4|. A dArsonval element 42 having a magnet 43, core 44, moving coil 45, and 4pointer 46, forms the reference junction of a thermocouple circuit similar to that illustrated in Fig. 1, in which thermocouple leads 41 and 48 are suitably connected to energize coil 45. The end of pointer 46 is visible from outside the case 4| through a transparent view plate 49 on which is scribed a curved hair line coinciding with the locus of movement of said pointer end. Mounted below the view plate 49 is a cylindrical chart scale 52 rotatably mounted on end brackets 53 journaled to receive a shaft 54 fixed axially to cylinder 52 by braces 55. One of the brackets 53 is provided with a projection 56 to which is affixed the inner end of a spiral bimetal element 51 the outer end of which is secured to 75 cylinder 52. The cylindrical scale 52 is marked with the equivalent of the unbroken curves of graph of Fig. 3 except that in place of rectangular coordinates the abscissae are curves corresponding to the curvature of the line 5| and the lines of constant relative humidity 58 are marked in per cent relative humidity at relatively close intervals.

The operation of the device is as follows: The pointer 46 is deflected from its zero position 59 in accordance with the depression of wet-air temperature below dry-air temperature. The bimetal element 51 is constructed and arranged to rotate the chart 52 in accordance with the dry-air temperature so that the ordinate corresponding to the ambient or dry-air temperature will always register with the line 5|. The ambient temperature may therefore be read at the point 59 of line 5| on the circumferential dry-air temperature scale of ordinates on the left side of chart 52. The end of pointer 46 will then indicate percent relative humidity quite accurately as correction of per cent relative humidity for change in atmospheric pressure from standard is practically negligible. An arrow 6| with other suitable marking on the outside of case 4| direct attention to the ambient temperature reading of the left hand scale of ordinates under point 59. The curved hair line 5| facilitates readings intermediate the lines 58. However, the scale 52 may be graduated to any degree of neness desired.

In case this instrument is desired to indicate the relative humidity existing at some remote point, the rotatable scale 52 may be actuated by a temperature-sensitive fluid-actuated helix 65 as illustrated in Fig. 6. With the wet-air unit in the air to be measured there is placed a bulb 66 connected by tubing 61 to the helix 65, an extension 68 thereof being xed to the bracket 53'. The free end 69 of helix 65 is attached to and arranged to rotate the axial shaft 54'.

In place of the wet-air unit illustrated in Fig. 1, greater current for the same wet-air depression may be obtained with the wet-air unit'l of Fig. 2, and thus make possible the use of a much less sensitive and therefore cheaper ammeter. In place of the element 34 of Fig. 1, there are shown in Fig. 2 three elements 14, each the functional equivalent of element 34. The thermocouples 15 are arranged as a thermopile with intermediate junctions 16 at the same temperature as the reference junction at ammeter 11. The elements 14 and attached thermocouples are electrically insulated from each other by shellac or other electric insulating coating which will not materially reduce the heat transfer to the evaporative envelope 18 from the elements 14.

Referring again to the graph of Fig. 3, I have shown corresponding respectively to the unbroken curves of constant relative humidity, an equal plurality of broken straight lines 89 diverging from a common point 8| on the axis of ordinates at about eight degrees Fahrenheit, one of said straight lines being drawn to coincide generally with each of the constant relative humidity curves between the ordinates 40'l F.' and 100 F. As is readily evident from Fig. 3, per cent relative humidity can thus be represented with good delity by a series of straight lines on a graph of dry-air temperature against depression of wetair temperature below dry-air temperature. In

order to read this approximately accurate per cent relative humidity directly it is merely necessary that the sensitivity of ammeters 36 or 11 be inversely proportional to the dry-air temperature above 8 F.

One method of accomplishing this is illustrated in Fig. 7, in which a dArsonval ammeter is comprised of a permanent magnet 86, core 01, moving coil 08, and pointer 89. In place of the customary scale, however, there is provided a resilient scale 90 composed of a fiat rubber band, or the like, graduated in percent relative humidity with the spacing of the graduations proportional to the horizontal spacing of the broken relative humidity lines of Fig. 3. The left-hand end 9| of the scale 90 is xed relative to magnet 86, While the right hand end 92 is attached to the temperature responsive or movable end 93 of a bimetal element 94 whose other end 95 is also xed relative to magnet 86. When suitably connected to one of the wet-air units previously described, the pointer 69 of the ammeter 95 will deflect proportionally to the depression of the wet-air temperature below ambient, or proportionally to the abscissae of the graph of Fig. 3. The element 94 is constructed so that the movement of its end 93, and therefore the stretch of the resilient scalev 90, in response to changes in ambient temperature, is such that for any ambient temperature between 40 F. and 100 F., the scale 90 will be stretched to correspond with the broken lines of the graph of Fig. 3 at the particular ambient temperature ordinate. Because the broken lines 80 are straight and converge to a. point this is a simple matter. The departure in Fig. 3, of the broken lines from the unbroken lines, between the ordinates 40 F. and 100 F., show that the errors to be expected in reading this meter directly in relative humidity are of a low order.

A pointer 96 is attached to the end 93 of bimetal element 94 and is arranged to traverse a scale 91 fixed with respect to magnet 86 and suitably graduated in degrees of temperature.

In place of the at band 90, I have shown in the relative humidity meter |00 of Fig. 8, an elastic scale |0| composed of a attened helical spring |02. The maior or cardinal divisions are provided with index tabs |03 extending beyond the' spring |02 and being marked with the appropriate value of per cent relative humidity.

A stiff internal core |05, suitably secured in fixed relationship to the frame |06 of the meter,

supports the spring |02 in the desired scale shape, in this case illustrated as a circular arc.

The left hand end |01 of scale |0| is secured to core |05. A bimetal element 94' has one end 95 fixed to the frame |06, with the other end 93' secured to the movable end |08 of scale |0|. The separations of the individual turns of spring |02 are preformed to correspond to aliquot subdivisions of pei` cent relative humidity in accordanc e with the separations of the broken lines of the graph of Fig. 3, taking into account the shape of scale |0|. A pointer |09 attached to the end |08 of scale |0| registers with an ambient temperature scale |0 xed to core |05 so that the meter canalsobe used as a thermometer.

As previously explained, per cent relative humidity can be indicated directly by the thermocouple circuits of Figs. l and 2 with fair accuracy provided the current sensitivity of ammeters 36 or 11 be inversely proportional to the dry-air or ambient temperature above 8 F. A convenient means for accomplishing this object is illustrated in Figs. 9 and 10, in which a pointer |20, attached to a moving coil element |2| moves over a scale |22. The element |2| is rotatably mounted in a guide bracket |23 fixed to the customary permanent magnet |24. the restoring torque and current supply being' by means of upper and lower spiral springs |25 and |26, respectively. A magnetic core |21 is supported between the poles oi the magnet |24, within the coil |2|, by a bimetal element |28 fixed to the magnet |24 at the equator part |29 opposite to the poles. The coil |2| differs from the usual construction in that it is considerably elongated and is mounted, in the embodiment illustrated, so that its top part is in the customary relation with its magnet and the elongated part extends downwardly well below the magnet. Vertical movement of the core |21, in response to changes in ambient temperature acting on bimetal element |28, varies the strength of the magnetic field in which coil |2| moves. The relative proportions of bimetal element |28,\ coil |2|, core |21, and magnet |24 are such that the sensitivity of the instrument varies in inverse proportion to the temperature above 8 F. As illustrated, th device is in the position of maximum sensitivity or lowest ambient temperature. On increase of ambient temperature, bimetal element |28 propels core |21 downwardly, resulting in decreased sensitivity.

In Fig. 11 I have illustrated a construction similar to that of Figs. 9 and 10 except that the bimetal element |28 is considerably longer than the corresponding element |28 of Fig. 9 and therefore movement of the core |21 is more sensitive to change in ambient temperature. Compactness of construction is attained by Wrapping element |28 about the equator end of the magnet |24. Similarly, the number of wraps may be increased to attain any degree of sensitivity required.

The desired relation of current sensitivity to ambient temperature above 8l F. can be conveniently produced in the ammeter by suitably varying the flux density of the magnetic fleld surrounding the movable coil. One method of varying the field strength at the poles is by providing a shunt path for the magnetic iiux. This is shown in Fig. l2, in which a field magnet |30 is provided with a keeper |3| swivelly attached by means of brackets |32 to the base |33 of one oi the poles |34 of the magnet. The free end |35 of keeper |3| is provided with a vertical extension |36 which engages a cam |31 pinioned to the base of the pole |38 opposite the pole |34. A lever extension |39 of cam I 31 is slidably linked to the movable end |4| of a temperature responsive bimetal element |40 which lies within the curve ci the magnet |30 with its fixed end |42v attached to said magnet adjacent the pole base |33. Movement of end |4| of element |40 in response to temperature changes rotates the cam |31 thereby positioning the keeper against the attraction of the magnet |30 to vary the air gap between keeper end |35 and pole |38.

AnotherA ux-shunting arrangement is illustrated in Fig. 13. A field magnet |50 is provided with a keeper |5| slidable perpendicularly to itself Within the enclosure of said magnet by means of a rack |52 attached to the keeper |5| and guided by a roller |53 on a iixed pivot |54. Engaging the rack |52 is a gear sector |55 rotatable about a fixed pivot |56 and provided with a lever extension |51. The outer end of lever |51 is pivotally attached, by a link |58 arranged to clear the top surface of the magnet, to the movable end |8| of a bimetal element |60 which is curved about the equator section of magnet |50 and is fixed to the base of the pole opposite said movable end. A change in temperature causes the bimetal ele- Y to the outer end 208 of shank 203.

- ment |60 to rotate the sector |55 and force the keeper |5| closer to or farther from the magnet poles, thereby weakening or increasing the ux density in the eld between the poles.

Flux density variation with temperature change may also be obtained with the magnetic circuit arrangement of Fig. 14, in which a eld magnet |65 is divided at the equator in two parts |66 and |61, the magnetic circuit being completed by a magnetic member |68 arranged for rotation with axis perpendicular to the planeof the magnet. The tw-o parts |66 and |61 of the magnet |65 are rigidly supported and held at the desired separation by U-shaped upper and lower `nonmagnetic brackets and |1| which are also adapted to house the bearings for the shaft |69 of member 168. Abimetalhelix |12 has its outer end fixed to a pin |13 in the bracket |1| and itsy inner end attached to the shaft |69 so that the magnetic element |68 will be rotated when the Y ambient temperature is changed. The shapes of cator, is illustrated in Figs. 17, 18, and 19. A

base 220 has attached thereto a miniaturecentrifugal blower 22| driven by an electric motor 222 which. is supplied With electricity via leads 223 and 224 from a suitable source of current supply (not illustrated) which may be A.C. or D.C. A collar bracket 225 is provided with anfoutward flange 226 which is attached by screws to the base f 220. A container 221 having a cylindrical lower the magnetic element |68 and of the cooperating f end surfaces of magnet parts |66 and |61 are such as to secure the desired relationship between the coil-field magnetic intensity and ambient tem.

perature.

Another exemplication of flux density varia-- tion by temperature-controlled movement of ay magnetic member uniting the two parts of a split i 'dArsonval instrument field magnet is shown in tioned by a non-magnetic yoke |83, so as to leave a space between said parts. `The magnetic circuit is completed by a magnetic member |84 arranged to be reciprocated in and out of said spacel by means of a rack |85 attached to member |84, said rack being guided by a `roller |86 on a fixed pivot |81, and said member |85 being aligned between guide |88 and guide |89, attached to the inner side of parts |8| and |82 respectively. A gear sector |90 rotatable about a fixed pivot |9| is provided with a lever extension |92 which is attached to the free end |93 of a bimetal element |94 by a pivoted link |95. Bimetal element |94 is curved about the inner periphery of the magnet |80 and has its fixed end attached near the equator end of the part |82 opposite said movable end |93. kChanges in temperature vary the relative position of element |84 in and out of closure of the space between parts |8| and |82, thereby increasing or weakening the field between the poles of magnet |80.

Temperature-responsive variation of field strength may also be attained by increasing thc length of the air gap between the poles of the field magnet of a d'Arsonval type instrument. In Fig. 16, this is accomplished in the magnet 200 by adapting one of the poles 20| to be movable toward or away from the conjugate fixed pole 202. The pole 20| is provided with a shank 203 reciprocable within an appropriate opening within the pole base 204 which is suitably enlarged and shaped to provide a guide therefor. The free end 205 of a bimetal element is provided with a slot 206 through which passes a screw 201 attached The fixed end of the bimetal element (not illustrated) is rigidly attached to the magnet 200, the movement of the bimetal element with temperature changes being such as to slide shank 203 within the pole base 204 and vary the length of the air gap and therefore its field intensity.

My invention, as embodied in a small self-contained compact portable relative humidity indiover the opening 23|.

reservoir` section 228, a duct rsection 229, and

bracket 225 which fits about the reservoir 228. The duct 229 communicates with the reservoir 226 by way of an opening 23| rimmed by an an` nular shoulder 232, the mouth 230 being centered A small wet-air thermocouple unit 233 which may be of the type shown in Figs. 1 or 2 or theequivalent thereof has its leads 234 and 235 extending through a cork, or rubber stopper 236 which ts snugly into the mouth 230 so that thebodyof the wet-air unit 233 is suspended centrallyfwithin the duct 229 and the dependent wick 231 of the unit 233 passes through the opening 23| into the reservoir 228. A conical collar 238 of waxed papery or other nonabsorbent materiaLis fitted over the Wick 231 so that said collar rests 0n the shoulder 232, thereby sealing reservoir 228 and minimizing the rate at which water is evaporated therefrom. The exhaust vent239 of the fan 22| abuts the inlet end of the'duct 229 and the joint is made airtight with an elastic sleeve 240 which ts over both parts. A dArsonval meter 24| constructed according to any of the modcations hereinbefore described asbeing adapted to read directly in percent relative humidity when connected to a properly-conditioned wet-air thermocouple unit, is also attached tothe base 220, the leads 234 and 235 of the wet-air unit being connected thereto.

The operation of this `meter is. as follows. The y reservoir 228 is filled with water 242 through the mouth 230 and opening 23|. The assembly comprising stopper 236, wet-air thermocouple 233, Wick 231, and collar 238 is slipped into place through the mouth 230. Electric current is then supplied to the motor 222 causing the fan 22| to force a stream of air through the duct 229 over the unit 233. In a few seconds the temperature of the measuring junction of the unit 233 will be reduced to the wet-air temperature and the meter 24| will indicate per cent relative humidity in accordance with the broken lines of the graph of Fig. 3, as previously explained.

The illustrations of Figs. 17 and 18 depict the container 221 as made of glass but it may be made of any other suitable material such as metal or a resin. In place of the electric motor 222, a spring motor 245 with manual wind 246, on a, base 220' illustrated in Fig. 20, may be used to drive the fan 22| which is shown with a screen or lter 241 to minimize accumulation of dirt in the duct and on the wet-air unit, although experience has shown that the wet-air unit will operate with good accuracy when quite dirty. The meter is designed for both continuous indication and for spot readings the fan is started and the indicating pointer of the meter 24| is observed until it reaches a minimum value of relative humidity, the steady state condition being reached in a few seconds as a. result of the small size of the wet-air unit.' Either of the wet-air units illustrated in Figs. 1 or 2 may be used with this meter, a less sensitive In the latter case I avancee ammeter being required if the wet-air thermopile of Fig. 2 is used.

As illustrated in Fig. 21, the direct reading relative humidity meter can be used to record relative humidity continuously on a strip chart 250. The chart '256 is driven by a motor 25| operating a. driving roller 252 and reroll 253. A marking pointer 254, suitably arranged to mark the chart 250, is actuated by a dArsonval element 255 which may be any of the elements previously described and illustrated in Figs. 9 to 16 in combination with wet-air unit 233, which may be either of the wetair units shown in Figs. 1 and 2, in the air stream from fan 22|. Similarly, a circular chart 250 illustrated in Fig. 22, may be used in place of the chart 256.

My invention is admirably adapted for relative humidity control, one method being illustrated in Fig. 23. A relative humidity meter 260, actuated by a dArsonval element arranged for response to relative humidity in accordance with the broken curves of Fig. 3 in conjunction with the wet-air unit 233 in the air stream from the fan 23|, as previously set forth, is provided with a metal pointer 26| traversing a scale 262. A bail 263 pinicned on the meter face on each side of the pointer 26| at points 266 and 265, is periodically oscillated so that it is alternately pressed against and released from the pointer 26| by an electric motor 266 which rotates a wheel 261 carrying an eccentric pin 268 engaging a radial slot in a crank 269 on the end of the bail 263. 'Ihe motor 266 is energized from an electric source 210 by means of conductors 21|. On the meter face, in a position to be electrically contacted by the pointer 26| when it is depressed by the bail 263, are three arcuate contacts 215, 216, and 211.

The contact 215, concentric with the axis of the pointer 26| extends across the entire path of the pointer so that it is contacted thereby in all positions of said pointer, The contacts 216 and 211 are arranged on adjacent arcs of a circle concentric with the contact 215 so that, as the pointer is moved from the left to the right end of the scale 262, the pointer periodically connects the contact 215 first with the contact 216, then the relative humidity rises to a point where the pointer 26| overlies the contact 211, 'the dehydrator will be operated to reduce the moisture content of the air. The space 218 can be adjusted to the characteristics of the conditioning apparatus to prevent hunting.

It should be realized that any of the foregoing meters are suitable for remote indication, recording, or control of relative humidity. It is Well-known that in place of the thermosensitive elements, generally shown herein as bimetal devices, remotely actuated thermosensitive devices, such as that of Fig. 6 or its functional equivaient may be used. Such construction permits the meter in its various forms to be used for air in ducts or other inacessible places and-the location of the meter itself to suit the convenience of the user.

The principle of the expansible scale in connection with my relative humidity meter is not intended to be limited to the use of an elastic medium. `In place of the elastic or resilient scales of Figs. 7 and 8, the scale may .be constructed in the manner of a folding fan or it may be constructed similarly to the iris of a camera, arranged to be positioned in accordance with the desired function of ambient temperature by a the free space 218 between the contacts216 and 211, and finally the contact 211. The space 218 is arranged to be centered at any position of relative huimidity and can be as wide as is desired. An electrically controlled humidifier 28|) of the aspirator or evaporative type, energized from the source 210, is arranged to be operated when the contacts 215 and 216 are connected. A step-down transformer 28|, with primary winding connected to the source 218, supplies low-voltage energy to the operating coil of a time-delay-reset relay 282 with instantaneous closing dtime-delay opening contacts, said coil being connected to the secondary winding of the transformer 28| in series with the contacts 215. and 216. The contacts of the relay 282 are arranged to close the electric circuit to the humidiler 280 on energizing the relay operating coil, the reset delay period of said relay being slightly greater than the interval be- -tween contacts of the pointer 26| with the contacts 215 and 216, so that the humidifier 280 will remain continuously energized while the pointer is in a position over the contact 216. When the pointer is over the space 218 for a period greater than the oscillation period of the bail 263, the

relay 282 resets and deenergizes the humidifier 280. In a similar manner, an electrically controlled dehydrator 290, such as of the silica gel or "terized in that said means is'constructed and f suitable bimetal element or the equivalent.

Although the invention has been described in considerable detail, such description is intended as illustrative rather than limiting, as numerous embodiments will be apparent to those skilled in the art. My invention, therefore, is not to be limited except insofar as is necessitated by the prior art or by the spirit of the appended claims.

I claim:

1. In a direct-reading relative humidity meter, a thermocouple circuit comprising (1) a wetair thermocouple unit adapted to be exposed under standard evaporating conditions to the air to be measured, (2) a reference thermojunction in said air, and (3) an electric instrument having a current-responsive element responsive to the current in said circuit, said instrument also including means to vary the current sensitivity of said current-responsive element as a function of temperature. Y

2. The invention set forth in claim 1 characterized by the addition of means for directing a current of the air to be measured against said Wet-air thermocouple unit.

3. The invention set forth in claim 1 characterized by the addition of means associated with said current-responsive Velement for producing a visible indication of the instantaneous position of said element.

4. The invention set forth in claim 1 characarranged to vary the current sensitivity of said element substantially in inverse proportion to 8F less than the dry-air temperature of the air to be measured.

5. The invention set forth in claim 1 characterized by the addition of means for directing a current of the air to be measured against said wet-air thermocouple unit and reservoir means for supplying moisture to said wet-air unit.

6. The invention set forth in claim 1 characterized in that said instrument is of the dArsonval type and said current sensitivity varying means varies the reluctance of the field magnet.

7. The invention set forth in claim l characterized in that said instrument is of the dArsonval type and said current sensitivity varyingmeans is bimetal powered to vary the reluctance of the iield magnet.

8. The invention set forth in claim 1 characterized in that said instrument is of the dArsonval type and said current sensitivity varying means is bimetal powered to move the magnetic core of the moving element .perpendicularly to the plane of the eld flux.

9. The invention set forth in claim 1 characterized in that said instrument is of the dArsonval type and said current sensitivity varying means is a bimetal-powered movable magnetic-iiux shunt.

10. In apparatus for relative humidity control, the combination of a thermocouple circuit comprising (l) a Wet-air thermocouple unit adapted to be exposed under standard evaporating conditions to the air to be controlled, (2) a reference junction, and (3) an electric instrument having a current-responsive element responsive to the current in said circuit, said instrument also including means to vary the current sensitivity of said current-responsive element as a function of temperature, a humidifier, a dehydrator, and means controlled by said current-responsive element for selectively render-v ing said humidifier and said dehydrator effective for humidifying or dehydrating said air, respectively, said last-named means comprising an electric circuit including an energy source', a relay with instantaneous closing time-delay opening contacts for controlling said humidifier and an associated set of contacts for connecting said source to energize said relay, a second relay with instantaneous closing time-delay opening contacts for controlling said dehydrator and a second set of contacts for connecting said source to energize said second relay, and an automatic mechanism constructed and arranged with relation to said sets of contacts at periodic intervals to close said first-named contacts when the position of said current-responsive element is below a certain range of movement of said element and to close said second contacts when its position is above said range, the timing of said mechanism and said relays being such that the contact-opening delay period of said relays is longer than the interval, whereby either relay, once energized, will retain its contacts in closed position if reenergized during the succeeding interval and will open its contacts if not so reenergized.

11. In apparatus for relative humidity control, the combination of a relative-humidity responsive element, a humidifier, an electric circuit including a relay with instantaneous closing timedelay opening contacts for controlling said aaeaoee humidirier, and automatic means for energizing and periodically reenergizing said relay within its contact-opening delay period as long as the position of said element is below a predetermined value of relative humidity.

12. In apparatus for relative humidity control, the combination of a relative-humidity responsive element, a dehydrator, an electric circuit including a relay with instantaneous closing time-delay opening contacts for controlling said dehydrator, and automatic means for energizing and periodically reenergizlng said relay within its contact-opening delay period as long as the position of said element is above a predetermined value of relative humidity.

13. In apparatus for relative humidity control, the combination of a relative-humidity responsive element, a humidier, a dehydrator, an electric circuit including a relay with instantaneous closing time-delay opening contacts for controlling said humidier, a second electric circuit including a second relay with instantaneous closing time-delay opening contacts for controlling said dehydrator, and automatic means for energizing and periodically reenergizing said ilrstnamed relay within its contact-opening delay period as long as the position of said element is below a predetermined value of relative humidity and for energizing and periodically reenergizing said second relay within its contactopening delay period as long as the position of said element is above another? predetermined value of relative humidity.

14. In a direct-reading relative humidity meter, a thermopile circuit comprising (1) a wetair thermopile unit comprising an evaporative envelope adapted to be subjected under standard evaporating conditions to the air to be measured, a plurality of thermocouple units electrically insulated from each other and enclosed within said envelope, the leads of said units constituting the sole means by which heat is introduced into the interior of said envelope, and said units and the thermal conductance of said leads being proportioned to the evaporative properties of the envelope such that said units are cooled substantially to the temperature of the surface of the envelope, (2) a reference and intermediate thermojunctions arranged to attain the temperature of said air, and (3) an electric instrument having a current-responsive element responsive to the current in said circuit and means to vary the current sensitivity of said element as a function of temperature.

15. In a direct-reading relative humidity meter, a thermopile circuit comprising (1) a wet-air thermopile unit adapted to be exposed under standard evaporating conditions to the air to be measured, (2) a reference and intermediate junctions arranged to attain the temperature of said air, and (3) an electric instrument having a current-responsive element responsive to the current in said circuit. said instrument also including means to vary the current sensitivity of said current-responsive element as a function of temperature.

FRANK KAHN. 

